The present invention relates to a magnetic recording medium, such as a magnetic disk and a magnetic tape, and a method of manufacturing the same.
Magnetic recording technology has a long history and is based on established techniques. The development of an information-based society in recent years has accelerated the development of a magnetic recording medium which has a higher density with improved properties. Such technology has thus been important more and more.
In particular, a thinner recording layer has been used for a higher recording density in the field of magnetic disks (e.g., hard disks and floppy disks) which serve as peripheral storage devices. The age of thin film medium using a magnetic thin film has been just beginning. In this event, important challenges are how to ensure the reliability of data stored thereon with a higher recording density and how to make such media suitable for mass-production at a low manufacturing cost.
It is known that a higher coercive force is achieved by using chromium as the underlying layer or film of a magnetic recording medium which has a magnetic layer of CoNiCr alloy or CoCrTa alloy (see, for example, IEEE Transactions on Magnetics Vol. MAG-3, No. 3 (1967), pages 205-207).
However, the magnetic CoPt alloy layer degrades orientation along the C-axis when it is combined with the underlayer of chromium (Cr) alone. This is because a crystal lattice constant of the magnetic CoPt-based alloy layer is greater than a crystal lattice constant of the conventional magnetic layer of the CoNiCr alloy or the CoCrTa alloy. As a result, the magnetic CoPt-based layer is not completely matched in atomic alignment with the underlying layer of the single component of Cr. Thus, this adversely affects the orientation of the C-axis.
In order to overcome this problem, U.S. Pat. No. 4,652,499 which is issued to Howard proposes to add a second or different metal to the Cr underlayer to improve the lattice constant. The orientation along the C-axis can be improved for the magnetic layer in film interface or boundary by means of changing the lattice constant of the alloy underlayer by addition of the different metal to the Cr alloy. This brings about an improvement of the coercive force or coercivity and the squareness.
The present inventors have found, as a result of detailed consideration, that the resultant recording medium has a significantly larger noise as a result of the addition of the different metal to the underlying Cr layer.
Another thin film recording medium is disclosed in Japanese Unexamined Patent Publication No. Hei 2-210614, namely, 210614/1990. The disclosed thin film recording medium consists of a magnetic layer of a cobalt-platinum (CoPt) based alloy having a high coercive force. In the thin film recording medium, the magnetic layer is divided into two magnetic films with a nonmagnetic intermediate film of chromium (Cr) interposed therebetween. This structure is helpful to reduce a noise during reproduction. More specifically, the resultant magnetic layer is composed of a first magnetic CoPt film, a nonmagnetic Cr film, and a second magnetic CoPt film, which are stacked in this order. It is also known that an underlying layer of Cr provides a favorable crystal structure for the magnetic CoPt film when placed under the thin magnetic film.
However, the above-mentioned magnetic recording medium which comprises the underlying Cr layer, the first magnetic CoPt film, the nonmagnetic intermediate Cr film, and the second magnetic CoPt film exhibits insufficient or unsatisfactory properties for a recording medium, such as coercive force, a reproduction output (product of residual magnetization and film thickness), and a signal-to-noise (S/N) ratio when it is manufactured with an in-line sputtering apparatus.
In addition, it has been found out that the magnetic recording medium having the above-mentioned structure is liable to a reduction in coercive force as compared with a single magnetic recording layer and is insufficient in over-writing properties.